A spin coating method is often used in the prior art as a method of coating an application liquid over sheet-like separate substrates such as large glass substrates typified by color filters for an LCD. Among spin coating methods, there is a method open to the atmosphere and a sealed cup method, but each of these methods has a low application efficiency of approximately 10% and moreover they have a disadvantage that the applied layer at corner portions of the substrate is too thick, and it has been pointed out that there will be problems concerning factors such as the amount of application liquid used, the layer thickness distribution, and the throughput as substrate sizes are expected to get larger in the future.
A knife coating method, a roll coating method, and a die coating method are available as methods capable of solving the above faults of the spin coating method. Each of these methods provides a clearance (gap) for application above the substrate on which the liquid is to be applied, the value thereof determines the application layer thickness, and smoothness is obtained for the application surface, but if the smoothness (degree of unevenness) of the substrate surface has a value greater than the accuracy of the application, it is difficult to achieve a uniform layer thickness by using any of these methods.
A dip coating method is generally known as an application method that is hardly affected by unevenness of the substrate surface, but it is difficult to avoid coating the non-application portions thereof and electively coat only the main surface of the substrate with this method. A liquid application method is known as a method with application principles that resemble those of the dip coating method and which also selectively coats only the main surface, as stated on page 336 of "Latest Advances in Coating Techniques," edited by Mr. Yuji Harasaki and published by Kabushiki Kaisha Sogo Gijyutu Center, Japan on Apr. 30, 1988. With this method, application liquid is supplied in substantially the horizontal direction through a die head or slide coating die, as will be described below. Liquid adjacent to a lip on the side surface of the die head or sliding die is moved in a direction perpendicular to the substrate that is to be coated. The application liquid supplied from the die head or sliding die forms an application liquid bead between the substrate and the die lip, and an applied layer of the liquid is formed on the main substrate surface in accordance with the upward movement of the substrate.
The above described application method is generally used as an application method for continuous band-shaped substrates. The thickness of the layer to be applied is determined by the relative speed of the substrate and the application head, and the viscosity of the application liquid. In other words, the application liquid pulled up from the bead portion droops down along the substrate under the force of gravity, and the thickness of the layer is determined by a balance between the speed at which the bead portion droops and the speed at which the substrate draws up the application liquid. This application principle is extremely similar to that of the dip coating method. However, if the application liquid has a low viscosity and a weak surface tension, it is difficult with this application method to form a bead in the clearance (gap) provided between the substrate and the application head and thus the application liquid is likely to fall under the force of gravity, without forming a bead, immediately after it is supplied, so that the clearance (gap) between the substrate and application head must be made extremely small in order to prevent these problems. However, with a substrate in which the degree of flatness is low with bumps of up to 100 .mu.m, such as a glass substrate used for an LCD color filter, differences in pull strength caused while the liquid is being pulled from the bead will occur, and thus differing shear forces will act on the liquid bead. If such locally differing shear forces are present, the thickness of the liquid layer drawn out from the liquid bead will not be uniform.
With this liquid application method, continuous liquid application to the main surfaces of substrates is possible. However, the main surfaces of the substrates extend vertically and liquid is supplied from the application head in a perpendicular manner with respect to the substrates, so that in the case of separate substrates, application liquid remaining in the bead portion will droop under the force of gravity as the edge portion of each substrate moves away immediately after the application. Furthermore, the drooping liquid flows around the edge portion of the substrate, reaching the rear surface thereof and the lower portion of the application head with resultant pollution. Therefore, this application method is not suited to application to separate sheet-like substrates.
Even further, the previously described liquid application method using a sliding die has the same defects as the above described liquid application method using an application head. In particular, with this method, the free flow of the application liquid leads to it flowing over the inclined surface of the sliding die and, since the application liquid bead is formed between it and the surface of the substrate, it is difficult to stop the supply at any desired point after the application. For this reason, this application method is not suitable for repeated application in sequence on separate substrates.
This invention is intended to solve the above problems with prior art techniques, and has as its objective the provision of a liquid application method and liquid application apparatus that can highly efficiently form a uniform liquid application layer in a stable state on a surface of a sheet-like separate substrate, irrespective of the properties of the liquid.